1. Field of the Invention
This invention relates to a reconfigurable beam-forming network to which a transmitter may be connected and, in particular, relates to a reconfigurable beam-forming network in which a plurality of distinct beams can be formed with power being fed to a plurality of regions being in-phase.
2. Description of the Prior Art
It is known to have reconfigurable beam-forming networks in which the shape of the beam can be varied. It is important, when the beam is varied, that no areas of the footprint are provided with less than satisfactory flux coverage and that the available flux can be concentrated, usually in a weighted manner within the footprint.
For example, in order to generate by means of a beam-forming network, a beam which covers the western half of Canada, a common approach is to use an array of electro-magnetic horns located in the focal plane of a parabolic reflector. In considering the antenna as a transmitting antenna, it is necessary to provide a control portion of the output of the transmitting source to each of the horns. This process, which provides the required weighting in amplitude and phase to each horn is referred to as beam-forming and is carried out by a beam-forming network. Usually, it is also necessary to provide coverage of the eastern half of Canada by means of a separate horn array and separate beam-forming network. Unfortunately, the region of Canada where the two half-Canada footprints touch, namely along the north-south dividing line of the West and East Canada beams, is subjected to low flux and special means must be taken to overcome these limitations. One known means employs dual-mode techniques which rely on the quadrature phase properties of directional couplers. Another means uses power sharing between single-mode beams. In using these techniques, transmitted power is fed principally into the beam-forming network forming the beam or footprint for West Canada and, at the same time, a small portion of the power is fed into the adjacent beam-forming network forming the beam for East Canada or into restricted parts of said beam-forming network. The restricted parts are usually those horns which are associated with the areas where the East and West Canada footprints overlap. If it subsequently becomes necessary that the transmitter power be transferred from the West Canada footprint to the East Canada footprint without loss of coverage in the overlap region, the overlap horns must also be connected into the East Canada array. This is usually accomplished by designing the overlap horns into a separate dual-mode subarray and beam-former that is fed by two ports, one of said ports being connected into the West Canada beam-former and the other being connected into the East Canada beam-former. In prior art beam-forming networks, where power is shared between single-mode beams, there is a power loss of approximately ten percent when the beam-forming network is in an East Canada or West Canada configuration. This power loss occurs at individual ground stations and is extremely expensive. A ten percent power loss can result in additional costs of one million dollars per channel at a ground station. When dual-mode prior art beam-formers are used for the overlap region subarray, phase weightings can no longer be uniform and a loss of antenna gain and beam-shaping control are therefore encountered.